Jellyfish are probably some of the most unusual and mysterious creatures that you'll ever encounter. With their gelatinous bodies and dangling tentacles, they look more like something from a horror movie than a real animal.

Jellyfish live mainly in the ocean, but they aren't actually fish -- they're plankton. These plants and animals either float in the water or possess such limited swimming powers that currents control their horizontal movements. Some plankton are microscopic, single-celled organisms, while others are several feet long. Jellyfish can range in size from less than an inch to nearly 7 feet long, with tentacles up to 100 feet long.